The present invention relates to a packaging machine for wrapping products in respective sheets of heat-seal wrapping material.
More specifically, the present invention relates to a packaging machine for wrapping products, each defined by one or more articles. In the following description, the articles considered are groups of rolls of paper, to which the following description refers purely by way of example.
Known machines for packaging rolls of paper normally comprise a wrapping unit for forming a tubular wrapping from a continuous web of heat-seal material, feeding groups of rolls of paper successively into the tubular wrapping so that each group of rolls is located between two free tubular portions of the tubular wrapping, and transferring the tubular wrapping and the groups of rolls to a sealing station.
The sealing station is equipped with a first group of sealing bars movable continuously along an endless first path extending in a vertical plane, and a second group of sealing bars movable continuously along an endless second path extending in said vertical plane and beneath the first path. The two endless paths comprise a common portion defining a sealing path, along which each bar in the first group of bars is fed in time with a corresponding bar in the second group of bars, so as to feed a respective group of rolls along the sealing path at a variable travelling speed.
Each bar has a hot sealing surface, which is heated electrically by a heating device with sliding electric contacts and comprising at least one guide of conducting material mounted parallel to the sealing path, and at least one brush, also made of conducting material, which is carried by the bar, is connected electrically to the sealing surface, and is engaged in sliding manner along the guide.
By combining the variable travelling speeds of the pairs of bars and, hence, of the groups of rolls along the sealing path, each free tubular portion is folded partly onto the respective groups of rolls, and is partly pressed between the respective sealing surfaces and stabilized by heat sealing.
Since the temperature of the sealing surfaces along the sealing path must be at least equal to a given sealing temperature, known packaging machines of the above type have several drawbacks, mainly due to the sealing surfaces being heated by electric current of constant intensity calculated experimentally and actually corresponding to a relatively high operating temperature which is normally higher than the sealing temperature.
Because of the high operating temperature, the sealing surfaces, which are brought into contact with each other to perform the respective sealing operations, undergo severe expansion and contraction, which result in relatively severe sliding friction and, hence, relatively severe wear of the sealing surfaces themselves.